This invention relates to a mill and particularly but not exclusively to a cane sugar mill for extracting juice from cane sugar.
Known cane mills comprise a mill housing including a pair of spaced upright frames in which three rollers are journalled for rotation about parallel horizontal axes which form or substantially form an isosceles triangle having a horizontal or substantially horizontal base. In such known mills the lower rollers, that is to say the two rollers traditionally called the feed and discharge rollers, are fixed, whereas the top roller is hydraulically movable toward and away from the base of the isosceles triangle to enable adjustment of the respective openings between the feed roller and top roller (hereinafter called "the feed opening") and between the discharge roller and top roller (hereinafter called "the discharge opening") during mill operation. The feed and dischage rollers are capable of adjustment when the mill is inoperative. Bagasse is transferred between the feed and discharge rollers by what is termed, inter alia, a trash plate disposed between the feed and discharge rollers and maintained in constant mesh with the feed roller.
In such a mill it is essential to provide a larger feed opening than discharge opening and specifically ratios varying between 3.5:1 and 1.7:1 have been used in practice, although a ratio of about 2:1 has been found preferable.
However, although initially the preferred ratio may be set this ratio will alter in the aforesaid mill disadvantageously during movement of the top roller toward or away from the base of the isosceles triangle.
Moreover, during operation of a mill having a preferred ratio of about 2:1, the reaction which the bagasse applies to the top roller is greater on the discharge side than on the feed side and it is believed that the resultant force thus applied to the top roller is inclined to the vertical on the feed side thereof by an angle of about 15.degree.. If the top roller bearings are housed for strictly vertical movement then such movement is hindered by the resultant force which tends to cause wedging of the top roller bearings in their respective housings during such vertical movement. In an attempt to minimize this disadvantage it is known that the position on the hydraulic ram is offset vertically and toward the feed side of the vertical center line of the top roller. Alternatively it is known that some mills are provided with top roller bearing housings inclined with respect to the vertical head of frames by some 15.degree. toward the feed side so that the top roller bearings move more or less in the same direction as the aforesaid resultant force. This does not, however, obviate the disadvantage, since the direction of the resultant force will vary as the ratio of feed opening to discharge opening varies from the preferred value. Furthermore, the ratio of feed opening to discharge opening will depart during movement of the top roller to a greater degree from the preferred constant value in such a mill.
A mill is also known in which the feed and discharge rollers are journalled in a lower housing while the top roller is journalled in an upper housing pivoted at one side of the mill to the lower housing and connected at the other side of the mill to a hydraulic ram which effects movement of the top roller in such a manner that the axis thereof describes an arc centered on the pivot axis of the upper and lower housings. By carefully locating the pivot axis it has been found that a substantially constant feed opening to discharge opening ratio can be achieved for the normal working range of the top roller. However, in practice, it is often desirable to modify the aforesaid ratio and this latter mill does not permit this to be done during operation of the mill.